[AArch64] Make legal shuffle generation consistent with how it is done for ARM

[AtariDreams]

Match what ARM does by building the legal vector shuffle and then rejecting it if building the shuffle fails.

[llvmbot]

@llvm/pr-subscribers-backend-aarch64

Author: AtariDreams (AtariDreams)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/81754.diff

1 Files Affected:

• (modified) llvm/lib/Target/AArch64/AArch64ISelLowering.cpp (+5-8)

diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
index 8c5a4cdae11634..bee423e2263b9b 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -11313,18 +11313,15 @@ SDValue AArch64TargetLowering::ReconstructShuffle(SDValue Op,
       LaneMask[j] = ExtractBase + j;
   }
 
-  // Final check before we try to produce nonsense...
-  if (!isShuffleMaskLegal(Mask, ShuffleVT)) {
-    LLVM_DEBUG(dbgs() << "Reshuffle failed: illegal shuffle mask\n");
-    return SDValue();
-  }
-
   SDValue ShuffleOps[] = { DAG.getUNDEF(ShuffleVT), DAG.getUNDEF(ShuffleVT) };
   for (unsigned i = 0; i < Sources.size(); ++i)
     ShuffleOps[i] = Sources[i].ShuffleVec;
 
-  SDValue Shuffle = DAG.getVectorShuffle(ShuffleVT, dl, ShuffleOps[0],
-                                         ShuffleOps[1], Mask);
+  SDValue Shuffle = buildLegalVectorShuffle(ShuffleVT, dl, ShuffleOps[0],
+                                            ShuffleOps[1], Mask, DAG);
+  if (!Shuffle)
+    return SDValue();
+  
   SDValue V;
   if (DAG.getDataLayout().isBigEndian()) {
     V = DAG.getNode(AArch64ISD::NVCAST, dl, VT, Shuffle);

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